Add change tracking fields to Artefact

This adds two new fields to the Artefact model, to contain the 
latest change note for a content item, and the public timestamp at
which it was last changed.

These will be included in the document that is sent to Rummager.
Merge pull request #251 from alphagov/add-public-timestamp-and-latest-change-note-to-artefact

Add change tracking fields to Artefact
Release version 24.2.0

This release includes the changes made in #251.
Merge pull request #252 from alphagov/release-24-2-0

Release version 24.2.0
Remove the `alternative_title` field from the `Edition` class

This field is no longer supported in the Publisher application now
that tag `release_1432` has been deployed.

This is the beginning of the clean up work to remove it from our
codebase.